The present invention relates to electronic photographic image forming apparatuses, such as copying machines, optical printers and facsimile machines using an electrophotographic system, and also relates to a transfer member cleaning method for them.
Transfer means of a conventional electrophotographic system operates in non-contact with a photosensitive body by discharging like a corotron. In recent years, however, from the viewpoint of transfer performance, there has been an increasing use of a contact transfer system which presses a belt or a roller as a transfer member against the photosensitive body, causes a transfer material in the form of a sheet, such as recording paper, to pass through between the photosensitive body and the transfer member, and transfers a transferable toner image formed on a surface of the photosensitive body to the recording paper by the function of a transfer bias (voltage) applied to the transfer member side.
However, this contact transfer system has problems, including direct transfer of the toner on the photosensitive body to the transfer roller which occurs when a document image is larger than the size of the recording paper; and extremely soiling of the transfer roller at the time of occurrence of a jam (paper jam), and strains on the back side of recording paper supplied thereafter and a transfer failure due to a substantial shortage of transfer bias voltage.
In order to solve such problems, Japanese Patent Application Laid-Open No. 1-319078 (1989) discloses a transfer roller cleaning method based on a reversal development system using a laser or LED. FIGS. 1A-1D are a timing chart of this transfer roller cleaning method. When the recording paper passes through a transfer position, a voltage of opposite polarity to the electric potential (FIGS. 1B, 1C) of toner is applied to the transfer roller (FIG. 1D) so as to transfer the toner to the recording paper. On the other hand, when no paper is passing, the surface potential on the photosensitive body (FIG. 1B) is made lower than that when forming an image, and further a voltage that has the same polarity as the electric potential of the toner and is higher than the surface potential (FIGS. 1B, 1C) of the photosensitive body is applied to the transfer roller (FIG. 1D).
In this transfer roller cleaning method, however, when no paper is passing, a voltage of the same polarity as the electric potential of the toner is always applied to the transfer member (transfer roller). Therefore, if an image forming apparatus does not have means for eliminating charges on the photosensitive body after a transfer process, there arises a phenomenon that the surface potential of the photosensitive body becomes closer to the voltage applied to the transfer member. Such a phenomenon causes a change in the image density when forming the next image, and therefore is not desirable. On the other hand, there are variations in the charge amount of each toner particle for use in forming an image, and not a little amount of toner of opposite polarity is included. For such toner which shows the charging behavior opposite to normal behavior, there is concern that the above-mentioned transfer member (transfer roller) cleaning method may rather cause heavy soiling of the transfer member.
Besides, Japanese Patent Application Laid-Open No. 2000-122450 proposes to enhance the ability to clean the transfer roller by applying a voltage generated by superimposing an AC voltage on a DC voltage as a transfer roller cleaning voltage. However, this method also has the same concern as that of the above-mentioned Japanese Patent Application Laid-Open No. 1-319078 (1989), and generation of the AC voltage itself causes an increase in the power supply cost.
Furthermore, although it is a method of changing the potential difference between the photosensitive body surface and the transfer member, Japanese Patent Application Laid-Open No. 10-282816 (1998) proposes to perform transfer cleaning without a transfer cleaning bias voltage, namely at a low cost, by causing the transfer roller to float and changing the applied voltage of a contact charging member when performing contact transfer cleaning.
In this technique, the surface potential of the photosensitive body periodically changes, and therefore complicated control is necessary to change a development bias voltage. Since the change in the surface potential of the photosensitive body relates to the time constant of the transfer roller, if there is a change in the environment of use, the condition of the transfer roller, etc., the time constant changes, and the cycle of change of the surface potential also changes. Thus, there is a possibility of occurrence of toner adhesion to the photosensitive body. Hence, although the purpose of this control is to clean the transfer roller, the control may rather cause soiling of the transfer roller eventually. Moreover, since there is a precondition that the charging means must be of a contact type capable of eliminating charges, non-contact type charging means cannot decrease the surface potential. In other words, the electric potential cannot be increased nor decreased. Other control of AC voltage is also described, but this control itself leads to an increase in the cost and does not meet the objective to reduce the cost.